1. Field of Invention
The present invention relates to an apparatus and associated methods for the in situ testing of engineering properties of soils and geo-materials. More specifically, the present invention relates to a mobile test system and associated test methods that determine in situ the stiffness and bearing capacity of soils and geo-materials.
2. Description of the Prior Art
The construction of civil infrastructure such as pavements, embankments, railway beds, airfield, walls, and industrial and commercial buildings typically requires a determination of soil engineering properties for construction verification and design purposes. These properties include the static or cyclic stress-dependent stiffness and bearing capacity of the soil. Although used for design, during construction these parameter values are not regularly measured as part of field quality control and quality assurance inspection processes due to lack of suitable field equipment and methods. Thus, construction processes are not field controlled to achieve the directly measured values provided in design documents. Instead, the construction processes rely on indirect measurement (e.g., California bearing ratio (CBR), density, dynamic cone penetrometer and falling weight deflectometer).
Recently, new design methods for pavement systems, for example, require selection of stress-dependent stiffness values of the pavement foundation layers to determine the pavement thickness. Soil stabilization and reinforcement via compaction and/or chemical and mechanical stabilization are being used to improve the in situ characteristics of soil and geomaterials to achieve the desired in situ stiffness. The in situ characterization methods typically rely on longstanding measurements of weight and volume relationships. However, the aforementioned methods do not characterize the static or cyclic stress and deflection dependent stiffness and bearing capacity.
It is known in the art to determine the in situ static or dynamic stress-dependent stiffness using deflectometers. For example, U.S. Pat. No. 4,116,041 to Tholen et al. discloses the use of a falling weight to transmit shock energy to a pressure plate engaged on the ground. Another example of in situ stiffness determination is U.S. Pat. No. 6,604,432 to Hamblem et al., which discloses the use of a portable apparatus that determines the shear modulus of the surface layer to determine soil compaction.
However, a disadvantage of these conventional test methods is that they (i) do not provide measurements under static conditions, (ii) only use a flat plate or a ring for transmitting the load to the ground, (iii) are only suitable for selected material conditions due to limits in the sensor systems, (iv) do not provide for confining stress control around the loading area, and (v) do not allow for strain or deflection controlled testing.
Therefore, a need exists for improved measurement techniques for determining in situ the stress and deflection dependent stiffness relationships and bearing capacity for a wide range of materials and conditions associated with various soils and geo-materials.